1. (Technical Field)
The present invention relates to a variable transmission-ratio steering apparatus capable of varying the transmission ratio of a steering force transmitted from a vehicle's steering wheel to vehicle's wheels to be steered.
2. (Related Art)
Most vehicles are provided with a variable transmission-ratio steering apparatus. One of such apparatuses is proposed by Japanese Patent Laid-open (KOKAI) publication No. 2001-278087.
This variable transmission-ratio steering apparatus has, in addition to an input shaft coupled to a vehicle's steering wheel and an output shaft coupled to vehicle's steerable road wheels, an electric motor, a lock pin, an excitation coil composed of a solenoid coil, and a controller.
Of these, the electric motor is driven in response to electric current to be supplied so that a driving force is transmitted to the output shaft, thus varying the rotation angle of the output shaft. The lock pin is disposed to mutually link, as one unit, the input shaft and the output shaft in a controlled connection/disconnection manner. The excitation coil is disposed to actuate the lock pin to stop the linkage between the input and output shafts. Moreover, the controller is in change of controlling the drive of both of the electric motor and the excitation coil.
However, in case that the excitation coil has abnormality which spoils its functions, the lock pin could not be actuated, in which the linkage between the input and output shafts is kept and no longer unlocked. In this situation in which the linkage is maintained, the controller operates to keep supplying current to the electric motor. This causes undesirable effects, such as overheat or burnout of coils, on the electric motor.